The implantation of biological materials, in particular those which are collagenous, is common in a number of medical applications. These applications include implantation of heart valves, pericardium, arteries, veins, skin, tendons, ligaments etc. The biological materials may be harvested from the same individual (autologous), from a different species (heterologous), or from the same species but from a different individual (homologous).
Without any prior treatment, implanted collagenous biological material that is heterologous or homologous may be regarded as a foreign antigen and trigger a host immune response that destroys the biological material. Treating the biological material with aldehydes such as formaldehyde, glutaraldehyde, glyoxal, or dialdehyde starch serves to: 1) enhance the mechanical durability and resistance to proteolytic attack by crosslinking collagen within the biological material; 2) greatly reduce an immune response to the implant by combining with and masking antigenic sites within the tissue; and 3) maintain sterility prior to implantation. Such aldehyde-treated material is termed "fixed."
While treatment with aldehydes prevents the triggering of an immune response and subsequent rejection of the biological material, residual aldehydes are known to be slowly released from aldehyde-treated biological materials and are known to be cytotoxic. The release of cytotoxic aldehyde related products can cause a local inflammatory response and can prevent complete healing of the implant. The customary method of briefly washing the aldehyde-fixed implant with sterile saline or water just prior to implantation cannot completely remove excess aldehyde within the fixed tissue because of diffusion limitations and because a substantial amount of aldehyde is released from the implant by slow hydrolysis over a long period of time. This limitation is particularly emphasized in thick aldehyde-fixed biological materials, such as stentless heart valves, vascular grafts, ligaments, and the like. Other methods are therefore necessary to further eliminate residual aldehydes thus "detoxifying" the fixed biological material.
One method of removing aldehyde residues is by using an aminodicarboxylic acid such as glutamic acid or aspartic acid as disclosed in U.S. Pat. No. 4,120,649 (Schechter). In addition to aminodicarboxylic acid, primary and secondary amines may be used singly or in combination to effect aldehyde removal as disclosed in U.S. Pat. No. 4,786,287 (Nashef et al.). The preferred amine is a primary amine having a general formula, R--NH.sub.2, where R can be an aliphatic, aromatic or a combination thereof which may be dissolved in a rinsing solution or immobilized on a solid support. The rinsing solution is buffered to a pH of 7.0 to 7.6. Because of the high pH of the rinsing solution, elevated temperatures and continuous rinsing or many rinsings are needed to promote efficient diffusion of the aldehyde from the tissue. This method, however, does not satisfactorily remove all the aldehydes within the tissue. For example, residual aldehyde remains at a level that prevents a cell lining from growing on the surface of thick-walled tissue.
A method disclosed in U.S. Pat. No. 5,188,834 (Grimm et al.) provides improvement over the aminodicarboxylic acid and amine treatment methods described above by using a dicarboxylic acid in an acidic medium with a pH of 2.5 to 5.5. At this lower pH, there is an enhanced degradation of tissue-bound polymeric aldehyde species to low molecular weight diffusable species. The low molecular weight species can more readily react with the dicarboxcylic acids and be removed from the tissue by diffusion. Grimm's method requires, however, that the aldehyde detoxified tissue be stored in a separate nonaldehyde containing storage solution. The storage solution typically contains two cytostatic agents, methyl and propyl paraben.
Thus, what is needed is another method for treating aldehyde-treated material, particularly thick tissue, in order to effectively remove residual traces of aldehyde from the material, and preferably, to provide a storage solution with improved bacteriocidal and fungicidal activity.